Certain metal powders (e.g., iron powder) are oxidized in the presence of air and moisture. Because the oxidation reaction is exothermic and generates heat, the metal powders have been incorporated into exothermic compositions to provide warmth. For example, conventional exothermic compositions contained a metal powder, activated carbon, and metal halide. The activated carbon acted as a catalyst to facilitate the exothermic reaction, while the metal halide removed surface oxide films on the metal powder to allow the reaction to proceed to a sufficient extent. Unfortunately, various problems existed when attempting to apply such exothermic compositions to a substrate. Specifically, if the exothermic composition were exposed to moisture during application, the exothermic reaction could occur prematurely. This ultimately would lower the quality of the exothermic composition and give rise to various other problems, such as an increased difficulty in handling due to coagulation.
Various techniques were developed in an attempt to overcome these and other problems. For example, U.S. Pat. No. 6,436,128 to Usui describes an exothermic composition that contains an exothermic substance, a water-absorptive polymer and/or tackifier, a carbon component and/or metal halide, and water. An excessive amount of water is used in the composition to suppress a premature oxidation reaction with air. Once formulated, the exothermic composition of Usui is laminated and sealed in a thin pouch. The pouch absorbs water from the composition so that, when the seal is broken, the exothermic reaction may proceed upon exposure to air and moisture. However, devices such as described above still possess a variety of problems. For example, certain components of the exothermic composition are often in particle form (e.g., carbon and exothermic substance). Unfortunately, these particles may rub off over time, thereby reducing the heat-producing characteristics of the device.
As such, a need currently exists for an improved technique for durably adhering an exothermic coating to a substrate that is simple, effective, and relatively inexpensive.